Most electro-optic devices utilize the convenient geometry of planar slab waveguides in their designs. Testing of these devices usually requires that a propagating laser beam be induced in the planar slab waveguide. A number of distinct types of propagating laser beams called modes can be directed by the waveguide. It is advantageous in testing to be able to selectively induce in the waveguide a particular mode of the propagating laser beam modes that the waveguide is capable of directing.
There exists three methods for coupling a free-space laser beam into a propagating laser beam in the slab waveguide. End-coupling requires that the edge of the waveguide be accessible, polished and adequately thick, and is not appropriate for inducing propagating laser beam modes in thin film planar slab waveguides. Also, alignment accuracy makes mode selection impractical.
Grating coupling entails the computation of complex grating structures which are etched onto the surface of the slab waveguide. A distinct grating structure must be computed and etched for each propagating laser beam mode that is required. Thus, grating coupling is a time consuming and complex process ill suited to applications involving experimental testing.
Prism coupling is the most experimentally flexible method for inducing a propagating laser beam mode in a slab waveguide. It is a surface coupling method suitable for thin film planar slab waveguides, and propagating laser beam mode selection is easily accomplished by varying the mode selection angle of the laser beam on the input surface of the prism. Also, prism coupling doesn't entail permanent damage to the surface of the waveguide as is required with the grating coupling method.
The efficiency with which prism coupling can induce a propagating laser beam mode in the slab waveguide is dependent upon the alignment of a number of prism coupling parameters. The following parameters should be adjustable: (1) the spacing between the coupling prism base and the waveguide, (2) the mode selection angle, and (3) the prism surface incident tilt angle. A zero tilt angle is optimum.
The practical utilization of the prism coupling method requires a convenient means of making these adjustments to the coupling parameters. The disadvantages of the method to date are its inflexibility, inconvenience and expense. Prism coupling requires fragile and expensive prisms of high refractive index. One of a kind housings for the prisms to effect the coupling are usually machined specifically for each experimental investigation. The prisms are generally glued into their housings committing them to that single experimental investigation and rendering them damaged and useless for subsequent use in other experimental investigations. In addition, a single experimental investigation may require coupling utilizing a range of prism sizes and shapes which is very inconvenient with present one of a kind machined housings that require selection of a particular prism size and shape.